Aim: What are Potential and Kinetic Energy?

Do Now:

A girl lifts a 10 kg object 1.5 m straight up. How much work has she done?

W = Fd

W = Fgd

W = (10 kg)(9.8 m/s2)(1.5 m)

W = 147 J

Energy

What is energy?

The ability to do work

Scalar quantity

Measured in Joules

Work – Energy relationship

The work that you put into a system is equal to the energy that system gains

Potential EnergyPotential EnergyStored energyStored energyGravitational Potential Gravitational Potential

EnergyEnergy – the energy stored – the energy stored due to objects vertical due to objects vertical positionposition

Measured from a specific Measured from a specific reference point (ground: h = reference point (ground: h = 0 m)0 m)

PE = mghPE = mgh

The heavy ram of a pile driver possesses potential energy

Back to the Do Now Problem

How much potential energy does the object have?

PE = mgh

PE = (10 kg)(9.8 m/s2)(1.5 m)

PE = 147 J

Work – Energy Relationship

If 147 J of work go into lifting an object, then the object obtains 147 J of potential energy

Change in Potential energy (loss or gain)

ΔPE = mgΔh

The 10 kg object from the do now falls 1 m. How much potential energy has it lost?

ΔPE = mgΔh

ΔPE = (10 kg)(9.8 m/s2)(1 m)

ΔPE = 98 J

Kinetic EnergyKinetic Energy

Energy of Energy of MotionMotion

KE = ½mvKE = ½mv22

ExampleExampleWhat is the kinetic energy of What is the kinetic energy of

a 1200 kg car moving at 100 a 1200 kg car moving at 100 m/s?m/s?

KE = ½mvKE = ½mv22

KE = ½(1200 kg)(100 KE = ½(1200 kg)(100 m/s)m/s)22

KE = 6 x 10KE = 6 x 106 6 JJ

What is the velocity of a 75 kg person running with 1,250 J of kinetic energy?

KE = ½mv2

1,250 J = (½)(75 kg)v2

1,250 J = (37.5 kg)v2

v2 = 33.3 m2/s2

v = 5.8 m/s

200 J of work are put into pushing a box across the floor.

How much kinetic energy has the object gained?

200 J

Work – energy relationship